1. Field of the Invention
The present invention relates to naphthalenetetracarboxylic acid diimide derivatives and an electrophotographic photoconductive material using the same. More particularly, the present invention relates to naphthalenetetracarboxylic acid diimide derivatives that include a specific substituent and have improved electron transporting ability. The invention also relates to electrophotographic photoconductive materials having electrical properties where the electrophotographic photoconductive material uses naphthalenetetracarboxylic acid diimide derivatives as an electron transferring material.
2. Description of the Related Art
The electrophotographic photoconductive material is used for electrophotographic image forming apparatuses including, for example, facsimiles, copying machines, laser printers, cathode ray tube (CRT) printers, light emitting diode (LED) printers, liquid crystal printers, and laser electrophotography. According to the basic working principle of the electrophotographic image forming apparatus, a photoconductive material is electrically charged and exposed to an image-forming light source to form an electrostatic latent image. Next, the latent image is developed using a toner by applying a development voltage. The developed toner image is transferred to a recording medium, followed by fixation of the transferred image.
The electrophotographic photoconductive material has an electrically conductive substrate, and a photoconductive layer that includes a charge generating material, a charge transferring material, a binder resin and other selective additives, where each of the materials are laminated on the electrically conductive substrate. The organic photoconductive layer is largely classified into a single-layer structure having a charge generating material (CGM) and a charge transferring material (CTM) dispersed in a single layer and a multi-layer structure having a CGM and a CTM, which are separately dispersed in a charge generating layer and a charge transferring layer, respectively. The single-layer structure of the photoconductive material is mainly studied for preparation of (+) type organic photoconductive materials while the multi-layer structure photoconductive material is mainly studied for preparation of (−) type organic photoconductive materials.
The (+) type single-layer organic photoconductive material generates less ozone that is harmful to humans and can be produced at a relatively low production cost since it has a single-layer structure.
The photoconductive layer of the (+) type organic photoconductive material includes an electron transferring material, a binder resin, and a charge generating material and may further include a hole transferring material. Since the electron transferring ability of the general electron transferring materials presently used is 100 times less than the hole transferring ability of the hole transferring material, the performance of the organic photoconductive material is largely affected by the electron transferring ability of the electron transferring material. Therefore, among the components contained in the photoconductive layer of the (+) type organic photoconductive material, the electron transferring material is the most important.
The electron transferring ability of the electron transferring material is affected by the solubility of the electron transferring material in an organic solvent and the compatibility to a polymeric binder resin. Therefore, the electron transferring material is selected by considering an organic solvent and a polymeric binder resin used in the photoconductive layer.
Examples of compounds commonly used as the electron transferring material include dicyano fluorenone, 2-nitro-9-fluorenone, 2,7-dinitro-9-fluorenone, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2-nitrobenzothiopene, 2,4,8-trinitrothioxanthrone, dinitroanthracene, dinitroacridine, dinitroanthraquinone, naphthoquinone, and 3,5-dimethyl-3′,5′-di-t-butyldiphenoquinone.
Since compounds having the dicyanofluorenone and diphenoquinone structures exhibit a weak electron transferring ability, an organic photoconductive material including an electron transferring material produced from these compounds may have a reduced charged potential and the exposure potential may increase upon use for a long period of time.
Compounds having a naphthalenetetracarboxylic acid diimide structure provide excellent electron transferring ability since they are much more soluble in organic solvents than compounds having a naphthoquinone structure. However, naphthalenetetracarboxylic acid diimide derivatives which have been developed so far are not sufficiently soluble in organic solvents and are not compatible with polymeric resins. Thus, there is a need for improvement of the electron transferring ability of the compounds.
U.S. Pat. No. 5,468,583 discloses a photoconductive material including a conductive layer, a charge generating layer, and a polymer binder layer having the electron transferring material dispersed therein, in which the electron transferring material has at least one cyclic bisdicarboxylic acid diimide compound represented by the following Formula 1:

In the Formula 1, Q1 and Q2 are respectively selected from a branched alkyl group, a non-substituted straight chain alkyl group, a substituted cycloalkyl group, an alkyl-substituted cycloalkyl group, a non-substituted straight-chain unsaturated alkyl group, or hydrogen atom, provided that both Q1 and Q2 are not hydrogen.
R1, R2, R3 and R4, respectively, are each a hydrogen atom, a C1 to C4 alkyl group, a C1 to C4 alkoxy group or a halogen atom.
The electron transferring materials disclosed in the above patent include naphthalenetetracarboxylic acid diimide derivatives as the electron transferring material. However, the naphthalenetetracarboxylic acid diimide derivatives are not sufficiently satisfactory in terms of the solubility in an organic solvent and are not compatible with the binder resin. Also, since they are apt to crystallize and form precipitates in the photoconductive layer and the electron transferring layer, electrophotographic properties of the photoconductive material may be deteriorated.
When naphthalenetetracarboxylic acid diimide derivative compounds are used in a concentration which can maintain compatibility with a binder resin, the photoconductivity of the photoconductive material may be deteriorated.
Japanese Laid-Open Patent Publication No. 2000-113504 discloses an optical recording medium including a naphthalene compound of the following formula:

In the Formula 2, A and B are independently an aromatic hydrocarbon group or a polycyclo group which may be substituted. The naphthalene compound disclosed in the above patent is used in an optical recording medium capable of recording and reading since it can be chemically modified by a laser having a wave length of 400 nm to 500 nm. Thus, the naphthalene compounds of the above patent are different from those for an electrophotographic photoconductive material.
Accordingly, there is still a need for the electron transferring material that can provide excellent electron transferring ability through improved solubility in organic solvents and compatibility with polymeric resins.